Lubricating oil compositions for internal combustion engines commonly comprise various combinations of chemical additives designed to impart improved performance characteristics to the lubricant and thereby the engine. The additives are commonly prepared as an additive package comprising a specific combination of additives for a particular application, which are mixed together with diluent oil. The diluent oil facilitates storage and use. To prepare a fully formulated oil, the additive package is mixed with the required base oil (s) and any additional additives.
An additive package can be stored on the shelf for some time between manufacture and use. Given that the additives comprise a variety of different chemicals, it is not unusual for some of the additives to interact with each other. Whilst the chemicals do not necessarily chemically react with one another, some of them do not mix well together. This can result in undesirable generation of haze or sediment in the additive package.
Additive package stability is a key concern to additive package formulators. Interaction of additives can limit the combinations of additives that the formulator can use and means that sometimes an additive combination that is desirable for lubricant performance benefits cannot be used due to additive package instability.
It has long been known to use friction modifiers and combinations of friction modifiers to obtain improved performance including improved wear performance and improved fuel economy. However, conventional friction modifiers often cause additive package instability as a result of poor compatibility of the friction modifiers with other additives present in an additive package. This effect becomes increasingly apparent as the amount of these conventional friction modifiers increases in the additive package. With the current drive to reduce friction coefficients of lubricants in order to improve fuel economy, it is desirable to use higher treat rates of friction modifier. However, this is not generally possible as it results in unacceptable levels of additive package instability.
In an attempt to address this problem, the present inventors have been looking for novel friction modifier compositions.
A recent example of a friction reducing additive for use in automotive engine oil and/or fuel is described in International patent application No. WO 2011/107739. The friction reducing additives described in this document are the reaction product of a hydrophobic polymeric subunit selected from polyolefins, polyacrylics and polystyrenyls and a hydrophilic polymeric sub unit selected from polyethers, polyesters and polyamides. The friction reducing additives described in WO 2011/107739 are said to facilitate improved fuel economy and fuel economy retention performance in an engine oil or fuel.